Basic peroxidases in isolated vacuoles of nicotiana tabacum L.

Vacuoles of tobacco mesophyll and of suspension-cultured cells were isolated in order to study the localization of peroxidase isoenzymes. Only basic peroxidases were detectable by electrophoretic separation of the vacuolar sap. Some of the basic peroxidases have formerly been described as an ionically bound cell-wall fraction. This fraction, however, was found to be an artifact produced by incomplete cell breakage. Reinvestigation of isolated cell walls confirmed that mainly acidic peroxidases are localized in the cell walls where they move freely or are bound. As a consequence of former and present results we think it probable that all of the peroxidase isoenzymes are secretory proteins because they have to be transported from the sites of synthesis in the cytoplasm to the sites of function, the extracytoplasmic spaces, cell wall (acidic peroxidases), and vacuole (basic peroxidases).Abbreviation ER endoplasmic reticulum - PAGE polyacrylamide gel electrophoresis     

An examination of the peroxidases from Lupinus albus L. hypocotyls

Peroxidases (EC 1.11.1.7) from hypocotyls of Lupinus albus L. cv. Rio Maior have been characterised using one- and two-dimensional, native electrophoretic techniques. Data are presented showing the complexity in charge and molecular size or shape of these peroxidases. We report the finding of a new acidic peroxidase and several new basic peroxidases in these hypocotyls, and of their stability to treatments considered to break ligand-induced variants and conformational variants derived from differences in polypeptide folding. Densitometric data demonstrate that these new peroxidases contribute up to 60 of the total peroxidase activity in hypocotyls. Studies of intercellular fluid, cell-wall and soluble fractions, with assays of purity were conducted in an attempt to define the subcellular locations of these additional peroxidases. The acidic form (pI 4.1) is greatly enriched in soluble fractions, three of the basic peroxidases (pIs 9.5, 9.7 and >9.7) are strongly associated to the cell wall, ad a minor, basic component (pI 9.7) is enriched in the intercellular fluid. Individual peroxidase activities with the substrates coniferyl alcohol, ferulic acid or indole acetic acid were compared by densitometric analysis of zymograms with those for guaiacol, and notable differences between these peroxidases in their capacity to oxidise indole acetic acid in vitro were identified. The possible functions of these peroxidases in vivo and their implications to current understanding of peroxidases in L. albus are discussed.Abbreviations APAGE anionic polyacrylamide gel electrophoresis - CA coniferyl alcohol - CPAGE cationic polyacrylamide gel electrophoresis - IEF isoelectric focusin - NEIEF non-equilibrated isoelectric focusing - 2D two dimensional - pI isoelectric point - RCPAGE reversed current polyacrylamide gel electrophoresis     

In vitro synthesis of glutathione peroxidase from selenite Translational incorporation of selenocysteine

The synthesis of glutathione peroxidase from [⁷⁵Se]selenite was studied in slices and cell-free extracts from rat liver. The incorporation of [⁷⁵Se]selenocysteine at the active site was detected by carboxymethylation and hydrolysis of partially purified glutathione peroxidase (glutathione:hydrogen peroxide oxidoreductase, EC 1.11.1.9) in the presence of [³H]selenocysteine and subsequent amino acid analysis. The synthesis of glutathione peroxidase in slices was inhibited by cycloheximide or puromycin and ⁷⁵Se was incorporated from [⁷⁵Se]selenite into free selenocysteine and selenocysteyl tRNA. Increasing concentrations of selenocystine caused a progressive dilution of the ⁷⁵Se and a corresponding decrease in glutathione peroxidase labeling. In cell-free systems, [⁷⁵Se]selenocysteyl tRNA was the best substrate for glutathione peroxidase synthesis. These results indicate the existence in rat liver of the de novo synthesis of free selenocysteine and a translational pathway of selenocysteine incorporation into glutathione peroxidase     

Peroxidase activity and isoenzymes in the culture medium of NaCl adapted tomato suspension cells

The medium of tomato (Lycopersicon esculentum) cells adapted to grow in the presence of 15 g l^–1 NaCl had a higher peroxidase activity than the medium of an unadapted tomato cell line. When the adapted cells were cultured in a medium without NaCl, the value found for peroxidase activity was intermediate. The increase in peroxidase activity was parallel to an increase of lignin-like compounds in the cell walls, as well as to an increased content or appearance of neutral and basic peroxidase isoenzymes. Apparently, the high values of peroxidase activity in the medium of the salt-adapted cells reflect the changed mechanical properties of the cell wall which, in turn, could be related to the salt adaptation process.Abbreviations LO Control tomato cell line unable to grow in the presence of 15 g 1^–1 of NaCl - L15 tomato cell line adapted to 15 g 1^–1 of NaCl and growing in this salt concentration - L15-0 tomato cell line adapted to 15 g 1^–1 of NaCl and growing in the absence of this salt - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA naphthaleneacetic acid - PBS phosphate buffer saline     

Production of peroxidases by hairy roots ofBrassica napus

Hairy roots cultures derived from leaf explants ofBrassica napus L. produced and secreted peroxidases. The enzyme activity in the medium increased with growth but it remained nearly constant in the tissue. The changes in extracellular peroxidase activity seemed to be correlated with the increase in a basic peroxidase of pI: 9.6. Four isoenzymes with pI in the range 8.5–9.6 and a neutral peroxidase of pI 6.3 were the most important peroxidases detected in cell extracts. Ca²⁺ addition at the beginning of the culture stimulated both the excretion of peroxidase to the medium and the enzyme activity in hairy roots but the isoenzyme profiles did not show qualitative changes during the growth cycle for both culture conditions.     

Characterization of basic <Emphasis Type="Italic">p</Emphasis>-coumaryl and coniferyl alcohol oxidizing peroxidases from a lignin-forming <Emphasis Type="Italic">Picea abies</Emphasis> suspension culture

A Norway spruce (Picea abies) tissue culture line that produces extracellular lignin into the culture medium has been used as a model system to study the enzymes involved in lignin polymerization. We report here the purification of two highly basic culture medium peroxidases, PAPX4 and PAPX5, and isolation of the corresponding cDNAs. Both isoforms had high affinity to monolignols with apparent K_m values in μM range. PAPX4 favoured coniferyl alcohol with a six-fold higher catalytic efficiency (V_max/K_m) and PAPX5 p-coumaryl alcohol with a two-fold higher catalytic efficiency as compared to the other monolignol. Thus coniferyl and p-coumaryl alcohol could be preferentially oxidized by different peroxidase isoforms in this suspension culture, which may reflect a control mechanism for the incorporation of different monolignols into the cell wall. Dehydrogenation polymers produced by the isoforms were structurally similar. All differed from the released suspension culture lignin and milled wood lignin, in accordance with previous observations on the major effects that e.g. cell wall context, rate of monolignol feeding and other proteins have on polymerisation. Amino acid residues shown to be involved in monolignol binding in the lignification-related Arabidopsis ATPA2 peroxidase were nearly identical in PAPX4 and PAPX5. This similarity extended to other peroxidases involved in lignification, suggesting that a preferential structural organization of the substrate access channel for monolignol oxidation might exist in both angiosperms and gymnosperms.     

Redox activity and peroxidase activity associated with the plasma membrane of guard-cell protoplasts

Redox systems have been reported in the plasma membrane of numerous cell types and in cells from various species of higher plant. A search for a redox system in the plasma membrane of guard cells was therefore made in efforts to explain how blue light stimulates stomatal opening, a process which is coupled to guard cell H⁺ efflux and K⁺ uptake. The rates of O₂ uptake by intact guard-cell protoplasts (GCP) of Commelina communis L., in the dark, were monitored in the presence of NAD(P)H since the stimulation of O₂ consumption by reduced pyridine nucleotides is used as an indicator of the presence of a redox system in the plasma membrane. Oxygen consumption by intact GCP increased two- to threefold in the presence of NAD(P)H. The NAD(P)H-stimulation of O₂ uptake was dependent on Mn²⁺ and was stimulated 10- to 15-fold by salicylhydroxamic acid (SHAM). Catalase, cyanide and ascorbate, a superoxide scavenger, all individually inhibited the SHAM-stimulated O₂ uptake. These are all characteristics of peroxidase activity although some of these features have been used to imply the presence of a redox system located in the plasma membrane. High levels of peroxidase activity (using guaiacol as a substrate) were also detected in the GCP and in the supernatant. The activity in the supernatant increased with time indicating that peroxidase was being excreted by the protoplasts. The properties of O₂ uptake by the incubation medium after separation from the protoplasts were similar to those of the protoplast suspension. It is concluded that our observations can be more readily explained by peroxidase activity associated with the plasma membrane and secreted by the GCP than by the presence of a redox system in the plasma membrane of the protoplasts.Abbreviations EDTA ethylenediaminetetraacetic acid - GCP guard cell protoplast - Mes 2-(N-morpholino)ethanesulphonic acid - SHAM salicylhydroxamic acid     

Macromolecule synthesis leading to cell division in Tetrahymena pyriformis after replacement of required amino acids

Summary Tetrahymena pyriformis W were brought to a nonmultiplying state by removal of required amino acids from their growth medium. After amino-acid replacement, the incorporation rates of H³-uridine, H³-thymidine and H³-leucine were measured by the autoradiographic method. Following amino-acid replacement, the first response was detected in RNA synthesis, then protein synthesis, then DNA synthesis and, lastly, in cell division. Amino-acid deprived cells showed a 23% net increase in DNA content, a result supporting the view of others that protein synthesis is not necessary for the initiation of DNA synthesis but is necessary for the maintainance of DNA synthesis.     

Thioredoxin reductase is one of the selenoproteins in both promyelocytic and granulocytic HL-60 cells

Human leukemia promyelocytic HL-60 cells differentiate into granulocytes when cultured with 1.25% dimethyl sulfoxide for 3 d. The radioactive Na₂ ⁷⁵SeO₃ incorporation and the amount of total proteins were interrelated in both promyelocytic and granulocytic HL-60. Promyelocytic cells had four times higher⁷⁵Se incorporation and 34% more protein synthesis than the granulocytic cells on the fifth culturing day. The enzyme activities of glutathione peroxidase (GSH-Px, E.C. 1.11.1.9) and thioredoxin reductase (TrxR, E.C. 1.6.4.5) in both types of cells increased significantly and approached steady stage on the third day. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) analysis and autoradiography of the proteins from the cells revealed three proteins with molecular weights of57, 28, and 21 kDa, respectively. These three⁷⁵Se-labeled proteins were present in both types of cells. The proteins from HL-60 cells were separated by DEAE-Sepharose and 2′5′-ADP-Sepharose columns. The purified 57-kDa protein had TrxR activity of 0.744 Μmol 5′-thionitrobenzoic acid (TNB) formed/min/mg protein and two isoelectric points at pH 5.9 and 6.0. These results suggest that TrxR is one of the selenoproteins in both promyelocytic and granulocytic HL-60 cells.     

Mutational and kinetic analysis of basic amino acid transport in the cyanobacterium Synechocystis sp. PCC 6803

Two nitrogen-deregulated mutants of Phanerochaete chrysosporium, der8-2 and der8-5, were isolated by subjecting wild type conidia to gamma irradiation, plating on Poly-R medium containing high levels of nitrogen, and identifying colonies that are able to decolorize Poly-R. The mutants showed high levels of ligninolytic activity (¹⁴C-synthetic lignin ¹⁴CO₂), and lignin peroxidase, manganese peroxidase and glucose oxidase activities in both low nitrogen (2.4 mM) and high nitrogen (24 mM) media. The wild type on the otherhand displayed these activities in low nitrogen medium but showed little or no activities in high nitrogen medium. Fast protein liquid chromatographic analyses showed that the wild type as well as the der mutants produce three major lignin peroxidase peaks (designated L1, L2 and L3) with lignin peroxidase activity in low nitrogen medium. Furthermore, in low nitrogen medium, mutant der8-5 produced up to fourfold greater lignin peroxidase activity than that produced by the wild type. In high nitrogen medium, the wild type produced no detectable lignin peroxidase peaks whereas the mutants produced peaks L1 and L2, but not L3, and a new lignin peroxidase protein peak designated LN. Mutants der8-2 and der8-5 also produced high levels of glucose oxidase, an enzyme known to be associated with secondary metabolism and an important source of H₂O₂ in ligninolytic cultures, both in low and high nitrogen media. In contrast, the wild type produced high levels of glucose oxidase in low nitrogen medium and only trace amounts of this enzyme in high nitrogen medium. The results of this study indicate that the der mutants are nitrogen-deregulated for the production of a set of secondary metabolic activities associated with lignin degradation such as lignin peroxidases, manganese peroxidases and glucose oxidase.     

Lignin synthesis and its related enzymes as markers of tracheary-element differentiation in single cells isolated from the mesophyll of Zinnia elegans

Mesophyll cells isolated from Zinnia elegans L. cv. Canary Bird were cultured for 96 h in a liquid medium containing 0.1 mg l^-1 -naphthaleneacetic acid and 1 mg l^-1 benzyladenine in which both differentiation of tracheary elements (TE) and cell division were induced, or in a medium containing 0.1 mg l^-1 -naphthaleneacetic acid and 0.001 mg l^-1 benzyladenine, in which cell division was induced but TE differentiation was not. Lignification was found to occur only in the former medium, fairly synchronously after 76 h of culture, 5 h later than the onset of visible secondary wall thickening. Changes in the soluble phenolics were not correlated with TE differentiation. Of three important enzymes which have been reported to play a role in TE differentiation, the activity of phenylalanine ammonia-lyase (EC 4.3.1.5) in the TE-inductive culture was higher than that in the control culture between 72 and 96 h of culture, when TE differentiation progressed and lignin was synthesized actively. O-Methyltransferase (EC 2.1.1.6) activity was higher in the control culture than in the TE-inductive culture, indicating that this enzyme was not a marker enzyme of TE differentiation. The activities of peroxidases (EC 1.11.1.7), one extractable and the other nonextractable, with CaCl₂ from the cell walls, reached peaks at 72 h (just before lignification) and 84 h of culture (active lignin synthesis), respectively, in the TE-inductive culture only, whereas the activity of soluble peroxidase showed a similar pattern of increase in the TE-inductive to the control culture. These results indicate that phenylalanine ammonia-lyase and peroxidase bound to the cell walls can be marker proteins for the differentiation of TE.Abbreviations OMT O-methyltransferase - PO peroxidase - PAL phenylalanine ammonia-lyase - TE tracheary element(s)     

Effect of L-cystine on macromolecular changes during spore and parasporal crystal formation inBacillus thuringiensis var.thuringiensis

High concentration of L-cystine (0.25%) when present in a glucose-mineral salt medium inhibited sporulation-specific events like protease production, calcium uptake and dipicolinic acid synthesis inBacillus thuringiensis var.thuringiensis. In addition, the enzymes of the Krebs cycle from aconitase onwards were completely inhibited by a high concentration of cystine. At a low concentration of cystine (0.05%), none of the above mentioned macromolecular changes were affected. Lipid synthesis monitored by [1,2¹⁴ C]-acetate incorporation into lipid as well as into whole cells was completely inhibited.     

Elicitation of peroxidase activity and lignin biosynthesis in pepper suspension cells by Phytophthora capsici

Cell suspension cultures of three varieties of Capsicum annuum L., each with a different degree of sensitivity to the fungus Phytophthora capsici, responded to elicitation by both lyophilized mycelium and fungus filtrate with a hypersensitive reaction. They showed the synthesis or accumulation of PR-proteins with peroxidase (EC 1.11.1.7) activity and the accumulation of lignin-like polymer (as measured by derivatization with thioglycolic acid). The cultivation medium was optimised for both plant and fungus growth in order to avoid any stress during their combination. The resistant pepper variety, Smith-5, showed a more intense response to the elicitor preparations than the sensitive varieties, Americano and Yolo Wonder. This was particularly evident when the cell suspensions were elicited with the filtrate. After elicitation, the cell walls thickened through the accumulation of lignin, as can be observed by staining microscope preparations with methylene blue. Elicitation also reduced the level of total peroxidase activity in the susceptible varieties, while such activity increased in resistant varieties, and was accompanied by de novo expression of acidic peroxidase isoenzymes in the extracellular and cell wall fractions. Of note was the PR protein of pI 5.7 showing peroxidase activity, which was induced by both elicitor types in the elicited cell suspensions of the resistant variety alone, making it a marker of resistance. The increases in the activity of these peroxidases in the resistant variety are in concordance with the accumulation of lignin observed 24 h after inoculation by both elicitors from the fungus. The possible role of these isoenzymes in lignin biosynthesis, used to reinforce the cell walls against fungal penetration of the cells, is discussed. These results are in accordance with those previously observed in plant stem sections.     

Protein synthesis by Beggiatoa alba B18LD in the presence and absence of sulfide

The interaction of sulfide oxidation and protein synthesis by Beggiatoa alba B18LD was investigated using the incorporation of radiolabeled leucine to estimate protein synthesis. Leucine was assimilated into whole cells in the presence of 6.1 mM acetate at a rate of 0.6 nmol · min^-1 · mg protein^-1, 43% of which was incorporated into the protein fraction. Protein synthesis by B. alba was unaffected by 1 mM sulfide, whether or not the cells had been preincubated with sulfide. B. alba oxidized radioactive sulfide to sulfur within 30 s of addition of the label, whether or not the organism was preinduced by sulfide. Furthermore, chloramphenicol, which inhibited protein synthesis, did not significantly inhibit sulfide oxidation by sulfide-induced or uninduced B. alba. This indicates that sulfide oxidation is a constitutive process. Enrichments of sulfur inclusions from B. alba B18LD that were analyzed by polyacrylamide gel electrophoresis demonstrated two enriched peptides with M_r values of 13,000 and 15,000. The 13,000 and 15,000 M_r peptide bands were more evident in cells grown in a medium containing sulfide than in cells from a medium lacking sulfide. Although sulfide did not increase the rate of overall protein synthesis, the synthesis of a few peptides was increased by the addition of sulfide to the growth medium. Among those, the 15,000 M_r peptide was one of the most distinctive.Non-standard abbreviations SDS-PAGE Sodium dodecyl sulfate polyacrylamide gel electrophoresis - PPO 2,5-diphenyloxazole - POPOP 1,4-bis [5-phenyl-2-oxazolyl]-benzene - BSS basal salts solution - BH Beggiatoa heterotrophic (medium) - BSO Beggiatoa sulfide oxidation (medium) - CM chloramphenicol - TCA trichloroacetic acid - M_r molecular mass     

Stimulation of extracellular polysaccharide synthesis in oat protoplasts by the host-specific phytotoxin victorin

The host-specific phytotoxin victorin (HV-toxin) stimulates mesophyll protoplasts of susceptible but not of resistant oat (Avena sativa L.) to produce an amorphous, ethanol-insoluble extracellular material which stains with Calcofluor white and aniline blue. Over a 24-h period incorporation of [¹⁴C]glucose into ethanol-insoluble products is maximally stimulated by 60 pg victorin/ml, whereas at 6 ng/ml initial rates of incorporation are higher but the protoplasts collapse. The extracellular material produced in response to victorin is solubilized by cold 4.4 N NaOH and by commercial laminarinase and pectinase. Incorporation of [¹⁴C]glucose into cellulose (material resistant to Updegraff's acetic-nitric acid reagent) is stimulated as much as incorporation into other wall polysaccharides, but cellulose constitutes less than 15% of the total victorin-stimulated incorporation. Synthesis of ethanol-insoluble material that can be digested by pronase, i.e. protein, is inhibited by victorin above 60 pg victorin/ml. Formation of extracellular polysaccharide is stimulated at concentrations of victorin which cause almost complete inhibition of protein synthesis, indicating that de-novo protein synthesis is not involved. Preincubation of protoplasts with inhibitors of RNA or protein synthesis prevents both extracellular polysaccharide synthesis and cell death in response to victorin. Although previous studies have indicated a link between calcium and the action of victorin, several compounds which interact with calcium do not influence this response to victorin.Abbreviations EPS extracellular polysaccharide - SCM 0.5 M sorbitol +10 mM CaCl₂+5 mM 2-(N-morpholino)-ethanesulfonic acid (Mes), pH 5.8 This paper is dedicated to the memory of our teacher and colleague, Martin H. Zimmermann (1926–1984)     

Biosynthesis of cell-wall polysaccharides in cultured carrot cells

Biosynthesis of the cell wall in carrot cells (Daucus carota L.) cultured in a synthetic liquid medium was studied by measuring the incorporation of radioactive glucose and myo-inositol (MI). When the cells were fed with [¹⁴C]glucose in the presence of 0.01% MI, the label soon appeared in the neutral sugars in the cell wall but little radioactivity was found in the uronic-acid residues even after a prolonged incubation. On the other hand, radioactivity derived from [³H]MI was found to be distributed among uronic acids and pentoses but not in the hexose residues in the wall. The data indicate that MI is an important intermediate for the synthesis of acidic sugars in the wall of cultured carrot cells.Abbreviation MI myo-inositol     

Effect of exogenous phenols on superoxide production by extracellular peroxidase from wheat seedling roots

Competitive and complimentary relationships of various peroxidase substrates were studied to elucidate the enzymatic mechanisms underlying production of reactive oxygen species in plant cell apoplast. Dianisidine peroxidase released from wheat seedling roots was inhibited by ferulate and coniferol, while ferulic and coniferyl peroxidases were activated by o-dianisidine. Both ferulate and coniferol, when added together with hydrogen peroxide, stimulated superoxide production by extracellular peroxidase. We suggest that substrate-substrate activation of extracellular peroxidases is important for stress-induced oxidative burst in plant cells.     

Role of protein and RNA synthesis in the initiation of auxin-mediated growth in coleoptiles of Zea mays L.

The kinetics of inhibition by protein- and RNA-synthesis inhibitors (cycloheximide and cordycepin, respectively) of indole-3-acetic acid (IAA)-induced elongation growth were investigated using abraded coleoptile segments of Zea mays L. Removal of the cuticle — a diffusion barrier for solutes — by mechanical abrasion of the outer epidermal cell wall increased the effectiveness of inhibitors tremendously. In an attempt to elucidate the role of growth-limiting protein(s) (GLP) in the growth mechanism the following results were obtained. The elongation induced by IAA was completely inhibited when cycloheximide (10 mol·l^-1) was applied to abraded coleoptile segments as shortly as 10 min before the onset of the growth response (=5 min after administration of IAA). However, when cycloheximide was applied after 60 min of IAA treatment (when a steady-state growth rate is reached), the time required for complete cessation of growth was much longer (about 40 min). Cycloheximide inhibited the incorporation of [³H]leucine into protein within about 5 min. Cordycepin (400 mol·l^-1) prevented IAA-induced growth when applied as shortly as 25 min before the onset of the growth response (=10 min before administration of IAA) but required more than 60 min for a full inhibition of steady-state growth. The incorporation of [³H]adenosine into RNA was inhibited by cordycepin within 10 min. It is concluded that, contrary to previous investigations with nonabraded organ segments, the initiation of growth by IAA depends directly on the synthesis of GLP. Moreover, the apparent lifetime of GLP is at least four times longer than the time required by cycloheximide to inhibit the initiation of growth by IAA. This is interpreted to mean that GLP is not present before IAA starts to act but is synthesized as a consequence of IAA action starting a few minutes before the initiation of growth. Interpreting the kinetics of growth inhibition by cordycepin in a similar way, we further conclude that GLP synthesis is mediated by IAA-induced synthesis of the corresponding mRNA which starts about 10 min before the onset of GLP synthesis. Inhibition by cycloheximide and cordycepin of IAA-induced growth cannot be alleviated by acidifying the cell wall to pH 4-5, indicating that these inhibitors do not act on growth via an inhibition of auxin-mediated proton excretion.Abbreviations CHI cycloheximide - COR cordycepin - GLP growth-dimiting protein(s) - IAA indole-3-acetic acid - mRNA_GLP mRNA coding for GLP     

Incorporation of 14C in the cyanobacterium Synechococcus PCC 6301 following salt stress

Synechococcus PCC 6301 synthesized sucrose as a compatible solute following hyperosmotic shock induced by NaCl. Initial rates of photosynthetic ¹⁴C incorporation were reduced following salt shock. Photosynthetic rates were comparable in cells enriched for glycogen (by growth in NO ₃ ^- -deficient medium) and cells grown in NO ₃ ^- -sufficient medium in the absence of osmotic shock. Incorporation of ¹⁴C was predominantly into the NaOH fraction and the residual acidic fraction in cells grown in NO ₃ ^- -sufficient medium, whereas incorporation was predominantly into the residual acidic fraction in cells grown in NO ₃ ^- -deficient medium. Following salt stress, ¹⁴C incorporation was initially into the ethanol-soluble fraction and the majority of tracer was recovered in sucrose. Carbon-14 was detected in sucrose in cells which had been enriched for [¹⁴C]glycogen prior to salt stress, inferring that glycogen can act as a carbon source for sucrose synthesis following salt stress. Changes in the specific activity of sucrose are consistent with an initial synthesis of sucrose from glycogen followed by synthesis of sucrose using newly fixed carbon, in response to salt stress.This work was supported by the Agricultural and Food Research Council.